Conventional dispersion models are analyzed to determine the factors that increase the risk of death during gas toxic release. The analysis is based in determining the toxic gas concentration as a function of the distance from the emission point. Thus, the injury in a person would depend on this concentration and the exposure time. It is shown here that the wind is an important factor to define the worst scenario. In addition, terrain characteristics substantially modify the gas dispersion. The results suggest that the worst scenario appears with calm wind and with stable atmospheric condition on rural terrain. The Pasquill-Gifford model in passive dispersion and Germeles model in gas dense dispersion tend to overestimate the concentration yielding safer estimations.